This invention relates to a power supply unit used for arc welding work.
Conventional arc welding machines are controlled for the welding output power through the selection of an output value in a fixed power range in accordance with the predetermined rule of operation of a differential amplifier or the like which has a feedback of the state of arc. The welding output power is also controlled through the detection of a transition from the state of arcing to the state of short-circuiting and vice versa, and the switching of the output waveform according to the result of detection.
As a typical example of the consumable electrode arc welding work, the CO.sub.2 welding work will be explained in detail. In this welding work, the stickout length is normally set from 10 to 20 mm. However, in welding a structure, of complex shape or a large structure which involves narrow deep sections, the tip of the welding torch cannot access the welding position in some cases. In these cases, it is inevitable to carry out the welding work with a stickout length as long as 50 mm, and the operation may encounter difficulty in maintaining a constant stickout length depending on the degree of expertise of the welding worker.
With a longer stickout length, the arc becomes unstable, which causes a deficiency of beads, resulting in a defective junction in that portion, or causes an uneven bead width or a too small average bead width, or causes insuffient melting.
In order to prevent the above-mentioned adverse inference on the result of welding and the condition of operation, the welding work with a longer stickout length is carried out with its welding voltage adjusted higher than, the usual condition. However, if the stickout length is short, while the welding output voltage is adjusted higher, such a trouble as melt-sticking of the wire to the electrode can emerge, and on this account it is necessary to re-adjust the output voltage each time the stickout length varies.
However, the foregoing welding power control based on the selection of an output value in a fixed power range in accordance with the predetermined rule of operation in response to the fed-back arc state, in which the selected output signal is in a one-to-one relation with the feedback signal, does not necessarily select the optimal output signal for stabilizing the immediate state of the arc, and this is a cause of instability of arcing. In addition, if the control loop gain is set too small, the power output cannot respond immediately to a change in the state of arc, or if the loop gain is set too large, the output power overshoots, making the arc more unstable.
Moreover, the welding power control based on the switching of the output waveform by detecting the transition between the state of arcing and the state of short-circuiting under the foregoing feedback control is subjected to a delay of waveform switching, resulting in an improper output waveform for maintaining the optimal arc, which causes the disturbance of the output power. Since the initial phase of short-circuiting cannot be predicted, the emergence of a small-scale short-circuiting cannot be brought to a positive short-circuit state, and this causes the emergence of sputtering and uneven transition to the short-circuit state because of a varying period of transition, resulting in a poor appearance of beads.
In the semi-automatic welding work, the operation needs to be halted for the output adjustment each time the butt length varies as mentioned above, which makes the welding work awkward and inefficient and also adversely affects the quality of welding. It is very difficult to measure from outside the stickout length of welding which goes on currently or the stickout length for the subsequent welding operation, and therefore the welding power must be adjusted on a trial-and-error basis, which imposes increased problems on the efficiency of work and on the degradation of welding quality caused by improper adjustment.